The use of piezo injection valves has the advantage with combustion engines that the fuel for each combustion cycle can be distributed over a plurality of precise portions, and that an improved mixing of fuel and oxygen can thus be achieved. The combustion process can in this way be designed for lower pollution and more efficiency.
In directly driven piezo injection valves in particular, the fuel throughput is adjusted in that a piezo drive (stack) coupled with the jet needle is subjected to defined charge/energy. The energy applied creates a proportional force at the drive which generates a deflection at a mechanically or hydraulically coupled jet needle, and so permits a regulation of the fuel throughput.
This places high demands on the charging/discharging electronics, which is usually implemented in the form of a DC-DC converter. The electronics must apply defined charge/energy to the capacitively operating drive of the piezo injector in a short time with high precision, robustness and efficiency, and with good repeatability, and must discharge it again in a defined manner at the end of the injection process. A large number of injections following closely on one another can thus be generated in each combustion cycle in order to create the most homogeneous possible fuel-air mixture.
The converter must feed a defined charge to the piezo injector in a short time under current control, and charge the load from 0 V to up to 250 V. At the end of the injection process, the load must also again be discharged from up to 250 V down to 0 V. For reasons of power loss, cost and efficiency, it is desirable to recover the highest possible proportion of the charge transferred back from the injector into the converter.
Two-quadrant switched-mode power supply units are usually used for this in order to transfer charge from the internal store of the converter into the load and back again into the converter under bidirectional operation.
EP 0 871 230 B1 discloses a device and a method for charging and discharging a piezoelectric element, as is illustrated in FIG. 1.
That document relates to a simple down-up converter BUCK/BOOST with a first upstream DC-DC converter DCDC1, a discharge path LINEAR DISCHARGE, and a selection circuit SELECTION, along with the capacitive load CPIEZO which represents the piezo element of an injection valve.
The down-up converter BUCK/BOOST is constructed as a half-bridge with a first transistor M1 to which a second transistor M2 is connected in series, while diodes D1, D2, for example in the form of substrate diodes, are connected in parallel with the transistors M1, M2, respectively. The connecting point of the two transistors M1, M2 is connected through a main coil LMAIN and an LC low-pass filter to the supply terminal of the capacitive load CPIEZO. The LC low-pass filter is formed with a filter capacitor CFILT and a filter coil LEMC, and is connected through a first shunt resistor RSH1, used for current measurement, to the ground terminal GND of the circuit.
The first DC-DC converter DCDC1 is formed in the illustrated embodiment as a fly back converter, but may however also be realized by other converter types. It includes a transformer, whose primary winding PW is connected on one side through an EMC (Electro Magnetic Compatibility) filter to the positive terminal of a battery voltage VBAT, and on the other side through a transistor M0, with which a diode is connected in parallel, to the ground terminal of the battery voltage GND. The one terminal of the secondary winding SW is connected through a diode DDCDC to the terminal that carries a positive voltage of an intermediate circuit capacitor CDCDC, and the other terminal of the secondary winding SW is connected to the ground terminal GND, to which the other terminal of the intermediate circuit capacitor CDCDC is also connected.
The supply terminal of the capacitive load CPIEZO is connected through a discharge transistor M7, a diode connected in parallel and a current measuring resistor RSP, which is connected in series with the discharge transistor M7 to the ground terminal GND.
The other terminal of the capacitive load CPIEZO is connected through a selection transistor M3 with a diode connected in parallel and a second shunt resistor RSH2, which is connected in series with the selection transistor M3 and serves to measure current, to the ground terminal GND of the circuit. The series circuit of the capacitive load CPIEZO with the selection transistor M3 and with the second shunt resistor RSH2 may have further such series circuits connected in parallel with it, which is regularly the case in the application for injection valves for combustion engines.
The first DC-DC converter DCDC1 creates a buffered intermediate circuit voltage VDC at the intermediate circuit capacitor CDCDC. While the piezo element CPIEZO is being charged, the down-up converter BUCK/BOOST operates, considered simply, as a step-down converter (buck mode), and as a step-up converter (boost mode) during discharge. During charging, a current is created in the main coil LMAIN by pulse-width-modulated switching-on of the first transistor M1. While the first transistor M1 is switched on, the diode D2 is at first in blocking mode, and the current in the main coil LMAIN rises in accordance with equation (1):
                              i          L                =                              1            L                    ⁢                      ∫                          u              ·              dt                                                          (        1        )            
The differential current in the main coil LMAIN during the phase when the first transistor M1 is switched on can be approximately described by equation (2):
                              di          dt                =                              VDC            -            VPIEZO                                L            MAIN                                              (        2        )            
During the time when the first transistor M1 is switched off, the diode D2 acts as a freewheeling path for the coil current, and the energy stored in the main coil LMAIN is removed by the flow of current into the capacitive load CPIEZO. The differential current through the main coil LMAIN in this phase can be approximately described by equation (3):
                              di          dt                =                              VDC            -            VPIEZO                                L            MAIN                                              (        3        )            
In accordance with equation (2), the fall in the current depends on the potential difference between the voltage VDC at the intermediate circuit capacitor CDCDC and the voltage VPIEZO at the capacitive load CPIEZO, which becomes smaller and smaller as the load voltage VPIEZO rises. The smaller the potential difference, the longer the time for the set current in the main coil LMAIN to develop. If VPIEZO gets closer to VDC, the charging current through the main coil LMAIN is limited by the nature of the system, and is pinched off. Only load voltages VPIEZO that are lower than the intermediate circuit voltage VDC can thus be reached.
When discharging the load, the down-up converter BUCK/BOOST operates, considered simply, as a step-up converter. The load functions as a voltage source for the converter, which is operated with pulse-width modulation, as when charging. While the second transistor M2 is switched on, a current is developed in the main coil LMAIN in accordance with equation (4). In this case, the diode D1 of the first transistor M1 is blocking.
                              di          dt                =                  VPIEZO                      L            MAIN                                              (        4        )            
During the phase in which the second transistor M2 is switched off, feeding back (recovery) of the energy stored in the main coil LMAIN into the intermediate circuit capacitor CDCDC takes place. In this case, the current flows out of the capacitive load CPIEZO through the diode D1 back into the CDCDC Equation (5) applies here. Diode D2 is blocking here.
                              di          dt                =                              VPIEZO            -            VDC                                L            MAIN                                              (        5        )            
An extended down-up converter BUCK/BOOST is described in DE 10 2012 204 576 A1 and illustrated in FIG. 2. The same components are given the same reference signs there as in FIG. 1.
In contrast to the down-up converter BUCK/BOOST of FIG. 1, the down-up converter BUCK/BOOST of DE 10 2012 204 576 A1 is formed with a full bridge which, in addition to the transistors M1, M2 of the first half-bridge, includes a second half-bridge with transistors M21 and M22 connected in series, wherein (substrate) diodes D21 and D22 are again connected in parallel with these transistors M21 and M22, respectively. The main coil LMAIN is connected between the connecting points of the respective transistors M1 and M2, M21 and M22, respectively, of the two half-bridges. The second half-bridge is connected in parallel with the filter capacitor CFILT, so that the first shunt resistor RSH1 can also be used as the current measuring resistor for the current when charging the main coil LMAIN.
Here again, a first DC-DC converter DCDC1 generates a buffered intermediate circuit VDC at an intermediate circuit capacitor CDCDC. The down-up converter BUCK/BOOST with two half-bridges operates during the charging and discharging, considered simply, as a flyback converter. During charging, a current is created in the main coil LMAIN by simultaneous pulse-width-modulated switching-on of the transistors M1 and M22. While the transistors M1 and M22 are switched on, the diodes D2 and D21 are at first in blocking mode, and the current in the main coil LMAIN rises in accordance with equation (6):
                              i          L                =                              1            L                    ⁢                      ∫                          u              ·              dt                                                          (        6        )            
The differential current in the main coil LMAIN during the phase when the transistors M1 and M2 are switched on can be described by equation (7):
                              di          dt                =                  VDC                      L            MAIN                                              (        7        )            
During the time when the transistors M1 and M22 are switched off, the diodes D2 and D21 act as a freewheeling path for the coil current, and the energy stored in the main coil LMAIN is removed by the flow of current into the capacitive load CPIEZO. The differential current in the main coil LMAIN can be described here by equation (8):
                              di          dt                =                              -            VPIEZO                                L            MAIN                                              (        8        )            
In accordance with equation (7), the development of current in the main coil LMAIN depends on the voltage VDC at the intermediate circuit capacitor CDCDC, but is independent of the load voltage VPIEZO. This has the consequence that, regardless of the level of the output voltage, energy may always be stored in the main coil LMAIN, which is then transferred during the freewheeling phase to the capacitive load CPIEZO. This permits the generation of output voltages VPIEZO that are higher than the intermediate circuit voltage VDC. The load voltage VPIEZO may thus become as high as desired, limited only by the dielectric strength of the components in use.
When discharging the capacitive load, the converter operates, similarly to when charging, as a flyback converter. The load functions as a voltage source for the converter, which is also operated with pulse-width modulation. While the transistors M2 and M21 are switched on, a current is developed in the main coil LMAIN in accordance with equation (9).
                              di          dt                =                  VPIEZO                      L            MAIN                                              (        9        )            
During the phase in which the transistors M2 and M21 are switched off, feeding back (recovery) of the energy stored in the main coil LMAIN into the intermediate circuit capacitor CDCDC takes place. In this case, the main coil LMAIN drives a current through the diodes D1 and D22 back into the intermediate circuit capacitor CDCDC. Equation (10) applies here.
                              di          dt                =                              -            VDC                                L            MAIN                                              (        10        )            
Now, with the two solutions described above, as a result of equations (4) and (9), the problem arises when discharging that the development of current in the main coil LMAIN depends on the load voltage VPIEZO, which becomes smaller and smaller as the load CPIEZO becomes increasingly discharged. The smaller the load voltage VPIEZO, the longer the time for the set current in the main coil LMAIN to develop. If the load voltage VPIEZO approaches a critical voltage, the discharge current is limited and pinched off. This has the result that the load cannot be fully discharged, and a remaining residual charge remains in the injector.
Until now, one of the solutions to this problem has been that at the end of the discharge phase, a linear current regulator or resistor LINEAR DISCHARGE is connected in parallel with the load, and the remaining charge converted to heat. The switching element M7 of the current regulator LINEAR DISCHARGE must have an appropriate regulation and protection circuit.
In particular, in the case of injectors with a high energy requirement and a high injection rate, this method increasingly creates problems through heating the electronics, since the remaining residual energy must be dissipated as lost power. The non-recovered charge must be supplied as additional energy by the intermediate circuit converter DCDC1. Additionally, restrictions must be accepted in terms of minimum spacing between sequential injection pulses due to the long delay/settling and activation times of the linear regulation path, and this impairs the performance of the overall system.
The synchronization of the discharge currents at the transition between the clocked and linear operating modes which can, among other things, impair desired sensor effects when closing the injector or make evaluation of the drive through sensors impossible, is also problematic.